you don't need multiple threads to run things concurrently (such as network I/O, communicating with subprocesses) e.g., I believe url-retrieve in emacs doesn't

you don't need multiple threads to run things concurrently (such as network I/O, communicating with subprocesses) e.g., I believe url-retrieve in emacs doesn't stop the world (because there is -synchroniously version too). async is a thing